The present invention relates generally to gas turbine engines, and, more specifically, to rotors or drive shafts therein.
An aircraft turbofan gas turbine engine includes in serial flow communication a fan, compressor, combustor, high pressure turbine (HPT), and low pressure turbine (LPT). The fan is joined to the LPT by one drive shaft, and the compressor is joined to the HPT by another drive shaft disposed concentrically around the first drive shaft.
In operation, air passes through the fan and is pressurized in the compressor, and mixed with fuel in the combustor and ignited for generating hot combustion gases which flow downstream through the HPT and LPT which extract energy therefrom for in turn powering the compressor and fan, respectively. Both drive shafts are subject to heating by the combustion gases, with the LPT drive shaft experiencing a lower temperature than the HPT drive shaft.
Accordingly, the drive shaft must be formed of suitable high temperature, high strength materials for carrying loads during operation at the elevated temperatures. Both shafts are subject to torsion loads as the turbines drive the corresponding fan and compressor. The pressure forces acting across the fan, compressor, and turbines place the corresponding drive shafts under tension during operation. Arid, the drive shafts are also subject to bending and gravity loads as the engine elastically deflects during aircraft movement.
The LPT drive shaft extends substantially the full axial length of the engine between the fan and the LPT and is therefore relatively long and slender and is subject to considerable torsional, bending, and tensile loads during operation at elevated temperature. The drive shaft is therefore made of a suitable high strength material which can maintain its strength at the elevated temperatures experienced during operation. At elevated temperature, creep strength is important which allows the drive shaft to withstand high loads at elevated temperature without undesirable creeping which would substantially limit the useful life of the drive shaft.
Accordingly, the LPT drive shaft is typically manufactured as a unitary, one-piece component of a suitable monolithic material such as a high strength nickel alloy for providing sufficient stiffness and strength, including creep strength, fatigue strength, and yield strength, for use in the hostile environment of a gas turbine engine. The opposite axial ends of the drive shaft are configured in suitable fittings for permitting mechanical attachment to the fan rotor at one end, and the LPT rotor at the opposite end. The fan fitting is typically accomplished by providing an external spline on the drive shaft which engages an internal spline in the fan rotor. And, the turbine fitting is typically configured as a radial flange at the end of the drive shaft which is bolted to a corresponding flange on the LPT rotor.
Furthermore, the LPT drive shaft must be dynamically balanced for reducing vibration during operation, which is typically accomplished by providing two or more annular balance lands integral in the drive shaft from which material may be precisely removed to balance the entire drive shaft.
The resulting LPT drive shaft is therefore effective for accommodating the various loads experienced during operation, and at elevated temperature, but has a substantial weight. Since an aircraft engine powers an aircraft in flight, engine weight is a primary design concern and should be as low as possible without comprising engine structural integrity.
Various forms of composite materials are being developed for use in aircraft gas turbine engines for reducing the overall weight thereof while maintaining suitable strength under the specific operating environment thereof. However, composite materials have various limitations which complicate their use in a gas turbine engine. For example, composite materials typically do not enjoy comparable strength with their monolithic metallic counterparts at the required mechanical joints in the engine such as bolted flange joints or spline joints.
Accordingly, it is desired to provide an improved gas turbine engine drive shaft having reduced weight while maintaining stiffness and strength at elevated temperature, and including suitable mechanical joints for connection with adjoining components.